1. Field of the Invention
This invention relates to copying machines of the type which utilize a photo-receptor sheet as a toner-transfer medium and which is advanced between supply and take-up reels during the copying cycles, and more particularly to such a machine in which the photo-receptor sheet is wholly contained within a highly compact module.
2. Description of the Prior Art
In many of the earliest xerographic machines, a photo-receptor drum, for example, a drum coated with selenium, was first charged and then exposed in accordance with the scanning of an original document to be copied. The drum was then developed by applying toner to the remaining charged areas on the drum, and finally the toner was transferred from the drum to plain copy paper. In other early machines, primarily those marketed by companies other than Xerox Corporation, the copy paper itself was coated with a photo-receptor such as zinc oxide. These machines did not utilize a separate drum, and it was the zinc oxide coating which was charged, exposed and then developed with toner.
The use of an intermediate drum allows the finished copies to be made on plain bond paper. But the per-copy cost is relatively expensive due primarily to the high cost of the drum. On the other hand, although the per-copy cost is much lower when zinc-oxide coated paper is utilized, the fact that the final copy sheet has a zinc-oxide coating is a serious disadvantage and objectionable to many users.
For these reasons, in order to reduce the per-copy cost and yet allow copies to be made on plain bond paper, several proposals have been made for what might be considered to be a sort of hybrid machine. Instead of using a selenium drum, a photo-receptor sheet is used for charging, exposing and developing, with the toner on the sheet then being transferred to plain bond copy paper. Typically, a zinc-oxide photo-receptor sheet is used as an intermediate web serving in the same capacity as a selenium drum. Due to the low cost of the zinc-oxide coated sheet which is used as the intermediate web, the per-copy cost is greatly reduced.
Unfortunately, a zinc-oxide coated sheet exhibits optical fatigue. At best, it can be charged and exposed only several hundred times before it is no longer useable. For this reason, any practical machine which utilizes an intermediate photo-receptor sheet is provided with a relatively long sheet, usually wound between supply and take-up reels.
There are two basic approaches which may be taken for advancing the sheet. In one approach, the photo-receptor sheet is advanced incrementally, and each incremental section of the sheet is used several hundred times in succession. After each section of the sheet has been used to make the maximum number of copies before it is no longer useable, the sheet is advanced so that a new section of it can be used during the next several hundred copy cycles. Following a single advance of the sheet from the supply reel to the take-up reel, with several hundred copies being made after each incremental advance, the sheet is replaced by a new one. In the other approach, the sheet is advanced incrementally during each copy cycle. After the sheet has been transferred from the supply reel to the take-up reel, it is transferred back in the reverse direction. Thereafter, successive copies are made once again as the sheet is advanced incrementally during each copy cycle from the supply reel to the take-up reel. Among the reasons for incremental photo-receptor belt advancement, for example of zinc-oxide material, is that it allows the photoreceptor belt to "dark adapt" before being reused. This eliminates copy quality variations which are quite noticeable on machines where a same frame is re-used several times per minute. After several hundred rewinds of the sheet, it is replaced by a new sheet.
One prior art patent which discloses an upwardly removable photoconductor is U.S. Pat. No. 3,883,240, which discloses a drum rather than a flexible photoconductor stored in a removable module. Some illustrative prior art patents which illustrate the use of intermediate webs as described above are: U.S. Pat. No. 3,737,230 dated June 5, 1973; U.S. Pat. No. 3,600,082 dated Aug. 17, 1971; U.S. Pat. No. 3,575,506 dated Apr. 20, 1971; and U.S. Pat. No. 3,617,124 dated Nov. 2, 1971.
In some of the machines of the type described above, when it is necessary to replace the photo-receptor sheet, a time-consuming service call is required. After the old sheet is removed, a new supply reel is placed in the machine and the leading edge of the photo-receptor sheet is threaded through the various rollers in the machine and attached to the take-up reel. Not only is this time consuming, but it is virtually impossible to train an operator to do the job himself even if a spare photo-receptor sheet on a supply reel is maintained on the user's premises. To overcome this problem, it has been proposed in the prior art, for example, in U.S. Pat. No. 3,617,124, to utilize replaceable cartridges so configured that threading of the sheet through the machine is not necessary. But this prior art machine is not practical for two reasons. First, the cartridge is not maintained stationary in the machine during a copy cycle, and instead reciprocates back and forth within the machine. This slows down the copy cycle. Second, it is necessary for the user to turn the cartridge around after the photo-receptor sheet has been advanced during successive copy cycles until the sheet is wound on the take-up reel. This is a serious disadvantage as it requires frequent user involvement in setting up the machine.